Batteries are often used to power portable electronic defibrillators. Portable defibrillators generate and apply a high-energy defibrillation pulse to the chest of a patient to cause the patient's heart to stop fibrillating and return to a normal rhythm. The pulses require high energy levels (up to 360 joules) and sometimes multiple defibrillation pulses are required to restore the patient's heart to a normal rhythm. Thus, defibrillators require large amounts of power during normal use. Because of the power requirements, portable external defibrillators generally use special battery packs to power the defibrillator. If the batteries that are being used by the defibrillator become depleted, the patient cannot be treated.
The power supply needs for portable defibrillators are distinct from many other portable devices in that the precision and urgency required is much greater, because a patient's life is often at stake. Urgency is often required because the chances that a patient's heart can be successfully defibrillated increase significantly if a defibrillation pulse is applied quickly. Thus, it is important that a defibrillator be able to resolve any power supply problems quickly with a minimum of distraction to the user.
Due to the critical nature of the power supply, some portable defibrillators have been equipped with backup batteries. Such defibrillators allow an operator to switch to the backup batteries if the original batteries fail. In the past, such systems have usually required the user to manually set switches to select one of the alternate power sources. Such systems have also required the user to manually track the battery maintenance with a gauge or other device. It is difficult in such systems for a user to keep track of which power source is most appropriate for use and when to switch from one source to another without affecting unit operation.
One particular problem is that switching of the power supply in a defibrillator can interrupt critical functions, such as monitoring of the patient's heart and the charging of the circuitry in preparation for applying a defibrillation pulse. Also, many defibrillators have special safety systems that monitor for out-of-tolerance voltages in the system and trigger a reset when out-of-tolerance voltages occur. Thus, power switching in such systems can accidentally trigger the safety reset of the system.
Some prior implantable defibrillators have attempted to address some of the power supply issues of defibrillators. One such device is shown in U.S. Pat. No. 4,323,075 to Langer, which discloses a method for battery failure compensation for a power supply used in an implantable defibrillator. As shown in FIG. 2 of Langer, a pair of batteries B1 and B2 are connected in series to provide the power for the defibrillator circuit. Two diodes, D1 and D2, are connected in parallel with each of the batteries B1 and B2, respectively. As described, one of the problems with the circuit is that if one of the batteries connected in series goes "dead," the current output by the series of batteries is limited by the output of the dead battery. This presents a serious problem in a defibrillator where a high current is often needed to charge the capacitor that provides energy for the defibrillation pulse. While one solution to this problem would be to provide switches for bypassing a dead battery, this solution is unacceptable because it drops the voltage of the series batteries by removing the voltage provided by the "dead" battery. Even a "dead" battery that is unable to deliver sufficient current is usually capable of adding at least some voltage to the overall level of the series battery circuit. The voltage provided by the dead battery, in addition to the voltage provided by the good battery(s), are often both required to operate a fibrillation detection circuit. In Langer, the diodes D1 and D2 allow the necessary current requirements for the charging circuit to be met by bypassing the current around a dead battery, while still allowing the voltage level from the dead battery to be included in the circuit for operating the fibrillation detection circuit.
Other implantable defibrillators have attempted to address the need to generally monitor the life of a defibrillator battery so that some warning can be given before the battery is completely drained. Such a device is shown in U.S. Pat. No. 5,292,348 to Saumarez et al., which discloses an implantable cardioverter/defibrillator and method employing cross-phase spectrum analysis for arrhythmia detection, and also in U.S. Pat. No. 5,063,928 to Grevis et al., which discloses an apparatus and method for detecting and treating cardiac tachyarrhythmias. Both of these patents show a power supply, such as a battery, and a signal line for monitoring the battery's condition. The signal line provides an end of battery life (EOL) signal to a microprocessor. The EOL signal is a logic signal whose status is indicative of the approach of battery failure in the power supply.
Still other implantable defibrillators have addressed the issue of the need to perform automatic battery maintenance within a defibrillator. One such device is shown in U.S. Pat. No. 5,690,685 to Kroll et al., issued Nov. 25, 1997, which discloses an automatic battery-maintaining implantable cardioverter defibrillator and method for use. Kroll et al. describe a device and method for performing automatic battery maintenance as particularly applied to an implantable cardioverter defibrillator. Batteries are maintained at a predetermined state of charge by addressing a problem internal to the battery itself, specifically that over time batteries can develop a high internal impedance or equivalent series resistance. As described, the voltage, current or other parameter from the battery is monitored to determine if the state of charge value is too low, in which case a battery loading maintenance cycle is switched into activation until the state of charge value improves.
While the above devices do address some of the power supply issues that arise for defibrillators, they do not address the issues of how and when to switch to backup power supplies. Neither do they make use of the features of the new "smart" batteries that have been developed to provide measurements of their own internal parameters and thus indicate when battery failure is approaching. Nor are they easily upgradable to make use of new battery technologies as they develop.
The present invention is directed to providing a method and apparatus that overcome the foregoing and other disadvantages. More specifically, the present invention is directed to providing a method and apparatus for automatic power switching in a portable defibrillator that makes use of the most recently available battery technologies and is easily upgradable.